linux-kernel - Re: [PATCH] vsprintf: avoid misleading "(null)" for %px

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Date:   Mon, 5 Feb 2018 21:03:05 +1100
From:   "Tobin C. Harding" <me@...in.cc>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Adam Borowski <kilobyte@...band.pl>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joe Perches <joe@...ches.com>,
        Kees Cook <keescook@...omium.org>,
        "Roberts, William C" <william.c.roberts@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Laight <David.Laight@...LAB.COM>,
        Randy Dunlap <rdunlap@...radead.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH] vsprintf: avoid misleading "(null)" for %px

On Mon, Feb 05, 2018 at 10:44:38AM +0100, Petr Mladek wrote:
> Hi,
> 
> I add people who actively commented on adding %px modifier,
> see the thread starting at
> https://lkml.kernel.org/r/1511921105-3647-5-git-send-email-me@tobin.cc
> 
> Just for reference. It seems to be related to the commit 9f36e2c448007b54
> ("printk: use %pK for /proc/kallsyms and /proc/modules").
> 
> 
> On Sun 2018-02-04 18:45:21, Adam Borowski wrote:
> > Like %pK already does, print "00000000" instead.
> >
> > This confused people -- the convention is that "(null)" means you tried to
> > dereference a null pointer as opposed to printing the address.
> 
> By other words, this avoids regressions when people convert
> %x to %px. Do I get it right, please?
>
> > Signed-off-by: Adam Borowski <kilobyte@...band.pl>
> > ---
> >  lib/vsprintf.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > index 77ee6ced11b1..d7a708f82559 100644
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> > @@ -1849,7 +1849,7 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
> >  {
> >  	const int default_width = 2 * sizeof(void *);
> >  
> > -	if (!ptr && *fmt != 'K') {
> > +	if (!ptr && *fmt != 'K' && *fmt != 'x') {

I don't know if it matters but with this it won't be immediately
apparent that a null pointer was printed (since zero could hash to
anything).

thanks,
Tobin.